Chloramphenicol-induced aplastic anemia in calves occurs only after oral administration of the drug. Administration of the drug by intravenous route does not induce the same severity of bone marrow damage indicating that gastric bacteria may be responsible for the chloramphenicol-induced aplastic anemia in calves after oral administration of the drug. Moreover, the severe bone marrow depression and aplastic anemia produced by oral adminstration of the drug appears to be reversible after stopping the drug treatment for six months. These studies indicate that chloramphenico-induced aplastic anemia in calves may be the result of a nutritional deficiency induced by oral antibiotic treatment.